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| Titel |
Effective LA-ICP-MS dating of common-Pb bearing accessory minerals with new data reduction schemes in Iolite |
| VerfasserIn |
Balz S. Kamber, David M. Chew, Joseph A Petrus |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093194
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| Publikation (Nr.) |
 EGU/EGU2014-7701.pdf |
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| Zusammenfassung |
| Compared to non-destructive geochemical analyses, LA-ICP-MS consumes ca. 0.1 μm of
material per ablation pulse. It is therefore to be expected that the combined analyses of ca.
200 pulses will encounter geochemical and isotopic complexities in all but the most perfect
minerals. Experienced LA-ICP-MS analysts spot down-hole complexities and choose signal
integration areas accordingly. In U-Pb geochronology, the task of signal integration choice is
complex as the analyst wants to avoid areas of common Pb and Pb-loss and separate true
(concordant) age complexity.
Petrus and Kamber (2012) developed VizualAge as a tool for reducing and visualising, in
real time, U-Pb geochronology data obtained by LA-ICP-MS as an add-on for the freely
available U-Pb geochronology data reduction scheme of Paton et al. (2010) in Iolite.
The most important feature of VizualAge is its ability to display a live concordia
diagram, allowing users to inspect the data of a signal on a concordia diagram as the
integration area it is being adjusted, thus providing immediate visual feedback regarding
discordance, uncertainty, and common lead for different regions of the signal. It can
also be used to construct histograms and probability distributions, standard and
Tera-Wasserburg style concordia diagrams, as well as 3D U-Th-Pb and total U-Pb concordia
diagrams.
More recently, Chew et al. (2014) presented a new data reduction scheme
(VizualAge_UcomPbine) with much improved common Pb correction functionality.
Common Pb is a problem for many U-bearing accessory minerals and an under-appreciated
difficulty is the potential presence of (possibly unevenly distributed) common Pb in
calibration standards, introducing systematic inaccuracy into entire datasets. One key feature
of the new method is that it can correct for variable amounts of common Pb in any
U–Pb accessory mineral standard as long as the standard is concordant in the U/Pb
(and Th/Pb) systems after common Pb correction. Common Pb correction can be
undertaken using either the 204Pb, 207Pb or 208Pb(no Th) methods. After common Pb
correction to the user-selected age standard integrations, the scheme fits session-wide
model U–Pb fractionation curves to the time-resolved U–Pb standard data. This
down hole fractionation model is next applied to the unknowns and sample-standard
bracketing (using a user specified interpolation method) is used to calculate final
isotopic ratios and ages. 204Pb- and 208Pb(no Th)-corrected concordia diagrams
and 204Pb-, 207Pb- and 208Pb(no Th)-corrected age channels can be calculated for
user-specified initial Pb ratio(s). All other conventional common Pb correction
methods (e.g. intercept or isochron methods on co-genetic analyses) can be performed
offline.
Apatite, titanite, rutile and very young zircon data will be presented, obtained using a
Thermo Scientific iCAP-Qc (Q–ICP–MS) coupled to a Photon Machines Analyte Excite 193
nm ArF Excimer laser with a novel signal smoothing device
Chew, D.M., Petrus, J.A., and Kamber, B.S. (2014); Chemical Geology, 363,
185-199.
Paton C., Woodhead J.D., Hellstrom J.C., Hergt J.M., Greig A. and Maas R. (2010);
Geochemistry Geophysics Geosystems, 11, 1-36.
Petrus, J.A. and Kamber, B.S. (2012): Geostandards and Geoanalytical Research, 36,
247-270. |
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